MRI volume coils and MRI phased arrays may be considered as two different types of coils. See, e.g., Lee R F, Hardy C J, Sodickson D K, and Bottomley P A. “A Lumped-Element Planar Strip Array (LPSA) for Parallel MRI,” Magnetic Resonance in Medicine 2004; 51:172-183. The volume coils can be made of a group of conductor elements that are tightly coupled together. The elements may be coupled either by mutual inductance and end-rings in a “birdcage” layout, or by mutual inductance and shielding in a transverse electromagnetic (“TEM”) resonator. See, e.g., Hayes C E, Edelstein W A, Schenck J F, Mueller O M, and Eash M., “An Efficient, Highly Homogeneous Radiofrequency Coil for Whole-Body NMR Imaging at 1.5T,” Journal of Magnetic Resonance 1985; 63:622-628; Vaughan J T, Hetherington H P, Otu J O, Pan J W, and Pohost G M, “High Frequency Volume Coils for Clinical NMR Imaging and Spectroscopy,” Magnetic Resonance in Medicine 1994; 32:206-218; Tropp J., “The Theory of the Bird-Cage Resonator,” Journal of Magnetic Resonance 1989; 82:51-62; Foo T K F, Hayes C E, and Kang Y-W, “An analytical Model for the Design of RF Resonators for MR Body Imaging,” Magnetic Resonance in Medicine 1991; 21:165-177. Volume coils can be used to generate a homogeneous magnetic field pattern in a low field, where the resonance wavelength may be much larger then an imaged object.
Phased arrays can include a group of loop or strip conductor elements that may be mutually decoupled from each other. The elements may be decoupled either by complex conjugate cancellation of impedance, by a minimum current that can be caused by high impedance at the ports of a coil, or by providing a certain distance between adjacent elements. See, e.g., Roemer P B, Edelstein W A, Hayes C E, Souza S P, and Mueller O M, “The NMR Phased Array,” Magnetic Resonance in Medicine 1990, 192-225; Wang J., “A novel method to reduce the signal coupling of surface coils for MRI,” Proceedings of the ISMRM 4th Annual Meeting, New York, 1996, p. 1434; Lee R F, Giaquinto R, and Hardy C J, “Coupling and Decoupling Theory and Its Applications to the MRI Phased-Array,” Magnetic Resonance in Medicine 2002; 48:203213. Such coils can be capable of achieving both a high signal-to-noise ratio (“SNR”) and a large field of view (“FOV”).
Volume coils may be characterized as coupled structures, and phased arrays may be characterized as decoupled structures. A decoupled structure may be described as a totally degenerated coupled structure. Coupled volume coils and decoupled phased arrays each may include a set of tuned resonators having a certain degree of degeneracy. The volume coil structures, although they may be tightly coupled, can exhibit some degree of degeneracy. Azimuth symmetry in volume coils may lead to a certain degree of degeneracy, such that the number of resonance frequencies can be about half of the number of the elements of volume coils. The phased array structure may not be completely degenerate, although total degeneracy may be approached because of, in part, a high impedance at the receiver port that may be caused by a preamplifier. This lack of degeneracy may not be present in some phased array structures having a low input impedance preamplifier. The degree of degeneracy can be used as a measure of coupling or decoupling of a coil structure. Therefore, a volume coil may not provide a completely coupled structure, and a phased array may not provide a completely decoupled structure. Environmental factors located outside of the coils, such as impedance of receiver ports and loading, can also affect the coupling/decoupling of coils.
In the absence of degeneracy, -coupled n-element resonators may have a maximum of n resonance frequencies, which can be referred to as n basic modes. When m (1<m<n) resonance frequencies are merged into one resonance frequency in an n-element coupled structure, such a structure may be referred to as having m-degree degeneracy, and the frequency merger can be referred to as mixing modes. Birdcage coils and transverse electromagnetic resonators can be referred to as having a two-degree degenerate structure, and decoupled n-element phased arrays can be described as having an n-degree, or totally degenerate, structure.
Systems and methods that include or utilize mixing modes, such as degenerate first and second circular modes of a birdcage coil, may employ two independent coils. The imaging behavior of such systems has been observed to be insufficiently detailed. See, e.g., Wong E C and Luh W-M, “A Multimode, Single Frequency Birdcage Coil for High Sensitivity Multichannel Whole Volume Imaging,” International Society for Magnetic Resonance in Medicine 7th Scientific Meeting, Philadelphia, Pa., USA, 1999, p. 165; Lin F-H, Kwong K K, Belliveau J W, and Wald L L, “Sensitivity Encoded Imaging From Multiple Mode Birdcage Volume Coil,” International Society of Magnetic Resonance in Medicine 10th Scientific Meeting, Hawaii, USA, 2002, p. 853.
The use of two completely degenerate modes in birdcage resonators was experimentally achieved in some birdcage coils having a small number of elements, and has been observed to provide improved imaging results. See, e.g., Tropp J, “The Hybrid Birdcage Resonator,” Society of Magnetic Resonance in Medicine 11th Scientific Meeting, 1992, p. 4009; Leussler C, Stimma J, and Roschmann P., “The Bandpass Birdcage Resonator Modified as a Coil Array for Simultaneous MR Acquisition,” International Society of Magnetic Resonance in Medicine 5th Scientific Meeting, Vancouver, B.C., Canada, 1997; Eagan T, Cheng Y C, Kidane T, Shvartsman S, and Brown R, “RF Eigenmodes: Circulant Theory and Matrix Applications,” International Society of Magnetic Resonance in Medicine 10th Scientific Meeting, Hawaii, USA, 2002, p. 164.
MRI coils, including volume coils and phased arrays such as those described above, may suffer from certain performance deficiencies. Such deficiencies may include a low signal-to-noise ratio, inefficient power consumption, and poor imaging behavior in parallel modes.